clk: iproc: fix bit manipulation arithmetic

A 32-bit variable should be type casted to 64-bit before arithmetic
operation and assigning it to a 64-bit variable

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Ray Jui <rjui@broadcom.com>
Fixes: 5fe225c1 ("clk: iproc: add initial common clock support")
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>